Rotary hammers are known which have a housing and a hollow cylindrical spindle mounted in the housing. The spindle allows insertion of the shank of a tool or bit, for example a drill bit or a chisel bit, into the front end thereof so that it is retained in the front end of the spindle with a degree of axial movement. The spindle may be a single cylindrical part or may be made of two or more cylindrical parts, which together form the hammer spindle. For example, a front part of the spindle may be formed as a separate tool holder body for retaining the tool or bit. Such hammers are generally provided with an impact mechanism which converts the rotational drive from an electric motor to a reciprocating drive causing a piston, which may be a hollow piston, to reciprocate within the spindle. The piston reciprocatingly drives a ram by means of a closed air cushion located between the piston and the ram. The impacts from the ram are then transmitted to the tool or bit of the hammer, optionally via a beatpiece.
Some hammers can be employed in combination impact and drilling mode or in a drilling only mode in which the spindle, or a forwardmost part of the spindle, and hence the bit inserted therein will be caused to rotate. In the combination impact and drilling mode the bit will be caused to rotate at the same time as the bit receives repeated impact. Such hammers generally also have a hammer only mode in which the spindle is locked against rotation.
Rotary hammers are known to have overload clutches in the drive train which transmits rotary drive from the motor to the spindle, or forwardmost part of the spindle. Such overload clutches are designed to transmit rotary drive when the transmitted drive torque is below a predetermined threshold and to slip when the transmitted drive torque exceeds the threshold. During rotary hammering or drilling, when working on materials of non-uniform hardness, for example aggregate or steel reinforced concrete, the bit can become stuck, which causes the torque transmitted via the rotary drive train to increase and causes the hammer housing to tend to rotate against the grip of the user. An overload clutch can slip and interrupt rotary drive to the bit at a torque threshold below that where a user may experience difficulty in controlling the hammer. Accordingly, the clutch must slip reliably at a predetermined torque throughout the lifetime of the hammer, even after sustained use of the hammer.
An overload clutch of this type is disclosed in EP 0552328, in which a pair of cooperating ratchet plates are urged into engagement with each other by a compression spring. When a predetermined threshold torque is exceeded, for example as a result of the drill bit becoming stuck in a workpiece, the ratchet plates can slip relative to each other against the action of the spring. However, known overload clutches of this type suffer from the drawback that at very high torque levels, the ratchet plates can be moved rapidly out of engagement with each other to the extremities of their permitted relative movement and then move rapidly back into engagement with each other, causing problems in controlling the tool.
Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art.